1. Field of the Invention
This invention relates to a composition and method for treating the matrix around a well to increase the productivity of the well. More particularly, the invention relates to a composition and method which are especially suitable for treating high-temperature hydrocarbon-producing sandstone and carbonate reservoirs, and which are useful for removing acid soluble clays and other siliceous fines and/or carbonate deposits from the matrix around a well. More particularly still, the invention relates to an acidizing composition whose rate of reaction on such clays and other siliceous fines and/or carbonate deposits is retarded.
2. Description of the Art
It is often desired to increase the permeability of a subterranean reservoir penetrated by well so that a fluid can flow more easily through the reservoir and into the well. Many reservoirs originally have narrow pore throats due to the nature and configuration of the reservoir rock. Such pore throats become plugged or partially plugged with clays and other siliceous fines and/or carbonate deposits, particularly in the matrix around a well, due to the concentrated radial flow of fluids therethrough. The plugging material can either shift position, swell, or otherwise change in character when contacted by fluids flowing through the reservoir.
Previously, various compositions have been proposed for treating subterranean reservoirs to increase their permeability. Among these are numerous acidizing solutions for removing clays and other siliceous fines and/or carbonate deposits from the matrix. Acidizing is an especially widely practiced process for increasing or restoring the permeability of subterranean reservoirs to facilitate the flow therethrough of formation fluids, including oil and gas, and injected fluids such as enhanced recovery drive fluids. Acidizing involves injecting into the reservoir an acid or acids, typically hydrochloric acid, or hydrochloric acid followed by hydrofluoric acid, in order to dissolve plugging or partially plugging carbonate deposits, or clays and other siliceous fines in the matrix, thereby opening and enlarging pore throats and other flow channels so that the effective porosity or permeability of the reservoir is increased.
One problem in acidizing even moderately high temperature reservoirs encountered in petroleum-producing wells is that the acid is rapidly consumed by the reacting material immediately adjacent to the wellbore before the acid can penetrate any significant distance into the matrix. Without adequate penetration into the matrix, the acidizing operation can be of little value in treating the reservoir.
Relatively high well temperatures (above 100.degree. F.) render the acidizing solution especially corrosive to well hardware. Corrosion inhibitors which effectively control corrosion at moderate well temperatures can become less effective and sometimes break down completely at elevated temperatures, such as above about 200.degree. F. Another difficulty with the known corrosion inhibitors, especially when used in the large quantities required in higher temperature wells, is their tendency to form insoluble solid residues upon decomposition, thereby damaging the reservoir permeability.
In view of these problems, the prior art acidizing methods are limited, as a practical matter, to the acidizing of reservoirs having temperatures on the order of about 200.degree. F. and less.
Many prior art references describe acidizing or other compositions for injection into reservoirs. As an illustration, compositions containing various amides or amide derivatives have previously been suggested for introduction into reservoirs as part of an acidizing or other treatment. One such composition is disclosed in U.S. Pat. No. 3,826,312 to Richardson which covers a self-neutralizing well acidizing composition containing hydrochloric acid and a slow reactive pH increasing material such as an organic acid amide, for example formamide and acetamide, which increases the pH of the solution to neutral after a relatively prolonged length of time after injection.
U.S. Pat. No. 3,892,275 to Lybarger et al. describes a thickened aqueous well treating fluid for gravel packing, fracturing, fluid diverting, selective plugging, and fluid displacement. The composition is an aqueous liquid containing a cellulose ether thickener, an acidifying material, and a slow reactive pH increasing material. The acidizing material may be hydrochloric acid, present in sufficient quantity to decrease the solution viscosity of the system after a selected time-temperature exposure. The slow reactive pH increasing material may be a lower organic acid amide, for example formamide, dimethylformamide, or acetamide, which increases the pH of the solution to neutral after a substantial length of time.
U.S. Pat. No. 2,265,759 to Lawton et al. discloses a method for treating a well with an acid, such as hydrochloric acid, and a wetting agent which intensifies the action of the acid in dissolving oil-saturated calcareous deposits. Suitable wetting agents include onium salts such as the amidols of coconut oil fatty acids.
U.S. Pat. No. 2,238,671 to Woodhouse describes a method for increasing the fluid flow from oil wells by injecting therein an acidizing solution comprising an aqueous solution of a readily hydrolyzable amide such as formamide, acetamide, hydroxy acetamide, methoxy acetamide, diglycol urea, and isobutoxy acetamide.
U.S. Pat. No. 2,824,833 to Cardwell et al. shows a hydrochloric acid acidizing solution containing a water-soluble gum thickener, such as karaya, tragacanth, and the like.
U.S. Pat. No. 3,179,171 to Beale, Jr. describes a hydrochloric acid acidizing solution containing a polymer such as polyvinylpyrrolidone.
In spite of the success of some of these aforementioned treatments, the need exists for a further improved well treating composition and method for use thereof to increase the permeability of a subterranean reservoir, especially a reservoir containing clays and other siliceous and/or carbonate deposits plugging or partially plugging narrow pore throats in the matrix around the well.
Accordingly, it is a principal object of this invention to provide a composition and method for increasing the permeability of a subterranean reservoir by removing therefrom plugging and partially plugging clays and other siliceous fines and/or carbonate deposits from narrow pore throats in the matrix around the well.
A further object of the invention is to provide an acidizing composition, optionally including a conventional corrosion inhibitor, and a method of using said composition wherein the rate of reaction of the acidizing composition on materials in the reservoir even at a relatively high temperature is retarded.
Another object of the invention is to provide such a composition and method wherein the depth of penetration of the acidizing composition into the reservoir even at a relatively high temperature is increased.
Still another object of the invention is to provide an acidizing composition and method which cause a minimum of corrosion damage to the hardware in the well through which the said acidizing composition passes when being injected into the said reservoir.
Yet another object of the invention is to provide an acidizing composition and method which cause a minimum of corrosion damage to the hardware in the well through which the acidizing composition passes into the said reservoir wherein a conventional corrosion inhibitor is rendered ineffective or partially ineffective due to high temperatures or other causes, such as a mutual solvent removing the inhibitor from hardware surfaces.
Yet another object of the invention is to provide an acidizing composition and method wherein the acidizing composition preferentially seeks and enters oil- and gas- bearing zones for stimulation, rather than water-bearing zones.
Other objects, advantages and features will become apparent from the following description.